1. Field of the Invention
The present invention relates to a common base circuit with output compensation, a current-to-voltage circuit configured with the common base circuit, and an optical receiver implementing with the current-to-voltage converter.
2. Related Prior Art
A Japanese Patent Application published as JP-H09-008534A has disclosed an amplifier with the common base arrangement. FIG. 8 shows a fundamental portion of the common base circuit disclosed therein. The amplifier 100 includes three (3) transistors, 101 to 103. The transistor 103 has the common base configuration where the emitter thereof receives the photocurrent generated in a photodiode by illuminated with an optical signal; the base thereof is fixedly biased; and the collector generates an output. The other two transistors, 101 and 102, have an arrangement of, what is called, the tandem connection, namely, the collector of the transistor 101 is directly connected to the base of the other transistor 102, while, the base of the transistor 101 is connected to the emitter of the other transistor 102. These two transistors, 101 and 102, connected in tandem operate as a load of the last transistor 103.
Another Japanese Patent Application published as JP-H11-205047A has discloses, what is called, a trans-impedance amplifier (hereafter denoted as TIA) used in an optical receiver that converts a photocurrent into a voltage signal. The TIA disclosed therein includes a transistor with the common base arrangement, a variable current source coupled with the emitter of the transistor, a load resistor, a fixed bias source for the base of the transistor, and a controller connected between the input and the output of the TIA to adjust the magnitude of the current generated in the current source depending on the output voltage.
Still another Japanese Patent Application published as JP-2009-246823A has disclosed a type of TIA. The TIA disclosed therein has a plurality of power supplies dynamically switched depending on the magnitude of the input photocurrent.
An optical receiver generally includes a photodiode (hereafter denoted as PD), and a pre-amplifier to convert the photocurrent into a voltage signal and amplifies this voltage signal. A TIA is generally applicable to such a pre-amplifier. A TIA has an arrangement including an inverting amplifier with high input impedance and a trans-impedance element, typically a resistor, connected between the input and the output of the inverting amplifier. In such an arrangement, a substantial portion of a current input to the TIA flows in the trans-impedance to cause a voltage drop thereat. Thus, the voltage drop, which may be evaluated by a product of the input current with the impedance of the trans-impedance element, becomes proportional to the input current.
The TIA with the arrangement above described is necessary to set the input impedance of the inverting amplifier high enough, which equivalently enhance the input capacitance of the amplifier and resultantly degrades the high frequency performance of the pre-amplifier.
Another type of the pre-amplifier of an optical receiver has been known as the common base circuit. The common base circuit receives the photocurrent generating in the PD at the emitter of the transistor, and outputs a voltage signal form the collector. The common base circuit has an inherent feature of the low input impedance, which may eliminate the influence of the input capacitance of the device. Moreover, the output of the common base circuit, which is drawn from the collector, has a phase same with that of the input; accordingly, the common base circuit may reduce the miller effect between the output and the input.
However, the common base circuit has a subject explained in FIG. 9 that shows a fundamental circuit of a conventional common base circuit. The common base circuit 200 includes a transistor 201 whose base is fixedly biased by a voltage Va determined by a ratio of resistance of two resistors, 202a and 202b; the collector thereof is connected to the power supply Vcc through a load resistor 205; and the emitter is grounded through the constant current source 206. The input current Iin is given to the emitter of the transistor 201, while, the output Vout thereof is given at the collector.
When no input current Iin is input, the current flowing in the transistor 201 and the resistor 205 is given by IE, substantially equal to the constant current determined by the current source 206. When a substantial current Iin is input, which flows into the current source 206, the current flowing in the load resistor 205 becomes IE−Iin. Thus, as the input current increases, which decreases the current flowing in the load resistor 205, the voltage drop by the load resistor 205 becomes smaller and the output level Vout approaches the power supply Vcc and saturates thereto.
FIG. 10 shows eye diagrams of the output Vout of the common base circuit 200 as varying the input current Iin from 100 μA to 2000 μA. As shown in FIG. 10, the cross point of the eye diagrams shifts to the higher level as the input current Iin increases. This is because the output Vout rises faster and saturates as the input current Iin increases. Thus, the conventional common base circuit 200 has an inherent subject that the output Vout degrades the shape thereof as the input current Iin increases.